(1) Field of Invention
The present invention relates to the preparation of halogenated aliphatic nitriles and in particluar to preparation of halogenated aliphatic nitriles by halogenating an aliphatic nitrile in the presence of a halogenation catalyst or initiator. More particularly, the invention relates to a method for removing hydrogen halide from a mixture of a hydrogen halide and a halogenated aliphatic nitrile in order to produce a product having low hydrogen halide content and improved storage stability.
(2) Prior Art
Halogenated aliphatic nitriles are generally prepared commerically by direct halogenation of an aliphatic nitrile or of a less completely halogenated aliphatic nitrile. The halogenation may be conducted as a liquid phase halogenation, a vapor phase halogenation or a liquid phase halogenation utilizing a gaseous halogenating agent. Reaction conditions disclosed in the art vary widely as to temperature, procedural steps, catalysts employed, halogenating agents employed and other reaction parameters.
Likewise, numerous means have been set forth for separating by-products and unreacted reactants. In liquid phase halogenations, hydrogen halide has been partially separated by heating with or without stripping gases to degas the reaction mixture and/or by a series of distillations in the presence of various solvents. An example of this is set forth in A. Karr, U.S. Pat. No. 2,463,629 wherein azeotropic distillation is employed.
Another means which has been used with both liquid and vapor phase reactions is to contact the reaction mixture, in liquid form, with water to remove water solubles, separate phases, dry the organic phase with a suitable drying agent such as calcium chloride then purify the organic phase by distillation. An example of this procedure and modifications thereof are shown in R. T. Foster, U.S. Pat. No. 2,375,545 and L. U. Spence, U.S. Pat. No. 2,283,237.
It is known, as shown in British Patent 522,835, that the presence of hydrogen halides, at least during the halogenation reaction, leads to the formation of excess residues. In the case of trichloroacetonitrile, these residues are principally triazines and polymeric derivatives thereof. We have found that the presence of small amounts of hydrogen halides in the final halogenated aliphatic nitrile product will lead to this type of residue formation upon prolonged storage, that rapid residue formation occurs if a halogenated aliphatic nitrile, contaminated with more than minor amounts of hydrogen halide, is stored in the presence of transition metal compounds, particularly in the presence of ferric ion, but that this degradation is significantly reduced when the hydrogen halide concentration is reduced below that normally found bound to the nitrile when recovered according to presently known processes.
Prior processes for preparing halogenated aliphatic nitriles, at some point in the processing cycle, generate hydrogen halide in amounts greater than that acceptable to produce a storage stable product. One known means for attempting to remove hydrogen halide from an halogenated aliphatic nitrile mixture is to heat the mixture to a temperature approaching the boiling point and strip the hydrogen halides off by known means, remove the gases and then utilize only the liquid phase as final product. The gas phase is condensed at low temperatures and either neutralized and discarded or recycled to the process. The problem with this procedure is that at temperatures utilized to degas the mixture, substantial amounts of the nitrile are also removed. Condensation techniques heretofore employed do not permit recovery thereof substantially free of the hydrogen halide.
Another separation procedure which has been employed is to distill the entire reaction mixture containing halogenated aliphatic nitrile and hydrogen halide, leaving only residues behind, and then condense the resulting vapors at low temperatures. This technique produces a condensate having higher levels of hydrogen halide than those acceptable for a storage stable product.
We have now found that when a reaction mixture containing a halogenated aliphatic nitrile and hydrogen halide is heated above what shall be hereinafter referred to as a second predetermined temperature, a vapor is provided in which the nitrile and hydrogen halide exist in uncombined or unassociated form, that when the resulting vapor is then condensed at a temperature above what shall be hereinafter referred to as a first predetermined temperature which is below the boiling point of the nitrile, the halogenated aliphatic nitrile is obtained as a condensate which has unexpectedly reduced amounts of hydrogen halide and significantly improved storage stability. However, if condensation is effected at a temperature below the first predetermined temperature, a nitrile product is formed which contains unacceptably high levels of hydrogen halide and which has limited storage stability. As used herein, all references to condensation temperature refer to the condensate temperature at the time condensate is separated from the gas phase, i.e., removed from the condenser.